Method of making a tau-square



Aug. 27, 1968 G. s. DOLGORUKOV 3,398,449

METHOD OF MAKING A T-SQUARE Original Filed May 15, 1963 5 Sheets-Sheet 1IN V EN TOR.

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7, 1968 G. s. DOLGORUKOV 3,398,449

METHOD OF MAKING A T-SQUARE Original Filed May 15, 1963 5 Sheets-Sheet 2Fa jg 17 52%INVENT1'OR. W

27, 68 G. s. DOLGORUKOV METHOD OF MAKING A T'SQUARE Original Filed May15 1963 3 Sheets-Sheet 5 INVENTOR.

United States Patent 3,398,449 METHOD OF MAKING A T-SQUARE Gregory S.Dolgorukov, Ferndale, Mich. (407 Fisher Bldg., Detroit, Mich. 48202)Continuation of abandoned application Ser. No. 288,577,

May 15, 1963, which is a continuation-in-part of application Ser. No.178,868, Mar. 12, 1962, now Patent No. 3,103,071, dated Sept. 10, 1963.This application Oct. 13, 1966, Ser. No. 586,568

7 Claims. (Cl. 29-529) This invention relates to geometric instrumentsand more particularly to an improved method of making a T-square adaptedfor use, particularly but not exclusively, with a drawing board. Thepresent application is a continuation of my copending application Ser.No. 288,577 filed May 15, 1963, now abandoned, which is acontinuation-in-part of my applications Ser. No. 178,868 filed Mar. 12,1962, for T-square, now Patent No. 3,103,071 granted Sept. 10, 1963, andSer. No. 855,826 filed Nov. 27, 1959, for Drafting Instrument nowabandoned.

The conventional T -square construction which has been used with successfor many years includes a wooden head and a wooden arm havingtransparent plastic edges. The head and the arm are usually connectedtogether with the aid of screws. Such screws must necessarily be rathershort since the combined thickness of the head and of the arm may be assmall as only /2. The plastic edges are usually secured to the woodenarm by a tongueand-groove construction. In order to provide a moresecure connection, a groove is also provided at the free end of the armand a tie strip made also of plastic is inserted and cemented therein,thus holding in place in a more reliable manner the plastic edgesprovided at both sides of the wooden arm.

Such a construction, generally, has been very successful. However, inthe use of such T-squares some diffioulties have been experienced withthe joint between the head and the arm. Small screws used for thepurposes of such joints usually do not have suflicient bearing area toprovide a reliable joint. This condition is aggravated still further bythe softness of the wood. As a result, in some T-squares such a jointbecomes loose, causing the arm to rock or move with respect to the head,making the entire T-square unreliable and virtually unuseable.Accordingly, many proposals of various supplemental screws or jointfastenings seeking to reinforce the joint, or to provide increasedbearing area for the connectors have been made, and such joints havebeen used with various degrees of success.

However, entirely different and much more serious difficulties withT-squares of the above described construction have developed Within thelast decade. Two particularly objectionable conditions have developed.First, it was observed that the guiding edges of the T- squares made ofplastic became less and less durable, showing a number of notches ornicks after a relatively short period of use. It is well appreciated bythose skilled in the art that even a single notch on the guiding edge ofa T-square or a drafting triangle is very annoying to the user and mayforce him to discard even a relatively new instrument. Therefore,because of the appearance of such notches, the useful life of trianglesand T-squares became 'objectionably short, greatly increasing the costof keeping drafting instruments in workable condition, as compared withthat of the past years. It was assumed that such notches and nicks areproduced by hitting the sharp edges of the instruments on some object.It was found, however, that even extreme care in using the T-square didnot substantially decrease this condition and that in some cases suchnicks were found even on new instruments. Still more serious was thefact that T-squares made of the best "ice available materials and ofwell proven construction and apparently with careful workmanship wouldsimply fall apart after a relatively short period of use. In schoolswhere T-squares receive somewhat rougher treatment than in professionaldrafting rooms, breaking or chipping off of the corners of the plasticedges at the free end of the arm and their separation from the Woodenarm in many T-squares occurred in some cases as soon as three or fourmonths after the first use, making the T-squares virtually unuseable.

Some attempts have been made to utilize in T-square constructionsplastic materials, and particularly acrylic plastic, by making the armof solid acrylic sheet cut to strips of proper size with the edges andends of each strip smoothened by milling to provide a fully transparentarm and connecting such arm to a wooden or plastic head with the aid ofscrews passed through holes drilled in one end of the arm similarly to awooden arm or an arm made of wood with plastic lined edges. It wasfound, however, that in addition to loosening of the head such T-squarefailed very easily by the plastic of the arm cracking through the screwholes. Such cracking resulted in many cases from origination of cracksin the holes because of tightening of the screws, with such cracksextending through the entire width of the blade when further loads wereimposed on the arm. While a T-square so failing can be repaired bycutting ofi the broken end of the arm and redrilling the holes, thisfact did not fully solve the problem. -In the first place, such repairwould have to be done in a shop having a drilling jig with the use ofwhich the original holes were drilled. Such operation obviously cannotbe performed by the user in whose hands the T- square was broken andwould require return of the T- square to the factory. However, even ifrepaired at the factory, such T-square would have a shortened arm andwould be unuseable 0n the sizes of drawings for which it was originallyintended.

In order to eliminate the necessity of connecting plastic to wood, andfor the reasons of less costly production it has been attempted to makeT-squares having arms produced from acrylic or styrene plastic by amolding process. Such arms being of one-piece construction did notpresent the danger of separation of plastic edges and appeared to haveproduced more durable edges. However, molded plastic arms lackedoptically fiat surfaces, and, therefore, produced distortion of theimage underneath and resulted in eye strain in prolonged use. Moreover,molded guiding edges also lacked the advantages of precision cut edges.However, when attempts were made to straighten edges of molded arms withthe use of milling or routing cutters, the above difiiculties of nickingand breakage of corners at the free end of the arm were again observed.

Attempts have also been made to produce T-squares having arms ofcombined wood and plastic construction but to make the plastic partthereof of a unitary construction to prevent separation of the edges.Such arms were made by an extrusion process to provide on their top andbottom surfaces wide but shallow grooves or recesses adapted to receivethin strips of wooden veneer cemented therein. A T-square with such armslooks virtually identical to the conventional wooden T-squares havingplastic edges. However, their wavy or Streaky surfaces and distortion oftheir transparency was even more objectionable than in the moldedT-square arms and their guiding edges, both cut and uncut, proved tohave substantially the same disadvantage as molded plastic arms.

I have found that the above difliculties resulted not from the decreasein the quality of workmanship but from the fact that the oldconventional construction that stabilized over a period of many yearswas suitable or advantageous only for the use of materials of that timeand particularly for producing T-squares of combined wood and cellulosenitrate plastic construction. Cellulose nitrate plastic is anexceedingly tough and virtually shatter-proof material. Conventionalconstructions using wooden arms with cellulose plastic transparentedges, except for the matter of loosening of the joint between the headand the arm, served very well and usable T-squares, of such aconstruction, 20 or 25 years old were not at all uncommon. However, thehigh inflammability of celluose nitrate plastics requiring extremeprecaution in storage and fabrication became more and more realized,causing stricter municipal regulations and higher insurance rates. Inad- 7 dition, the. susceptibility of cellulose nitrate to attack byapparently contagious fungus disease greatly increased as time Went on.

Such disease usually manifests itself by an unsightly mold or by sweaappearing on the surface of drafting instruments, in the case ofT-squares on their plastic edges. Within a short time such mold developinto a rather unsightly white growth followed by cracking or crazing ofthe edges throughout their entire length. The sweat appears in the formof a thick liquid precipitate on the surface of the instrument andsimilarly produces cracks. In addition, presence of sweated cellulosenitrate instruments next to metal drafting instruments, such as draftingcompasses, ruling pens, and the like, is very corrosive, and, therefore,damaging to such instruments. As a result, the use of cellulose nitratefor drafting instruments has been virtually discontinued, and otherplastic materials, particularly acrylic plastic, have been used for suchapplication for quite a number of years.

I have found that the experienced lack of durability of the conventionalT-squares does not result from mechanical causes incidental to their usebut is imparted to the edges of T-squares and other drafting instrumentsin the process of their smoothening with milling or routing cutters. Ihave also found that separation of the plastic edges from the wooden armresults not from the shortcomings of the cement but is due to thefailure of the ends or corners of the plastic edges at the free end ofthe arm. Such failure occurs even from a very slight impact causing thecorners to break off and the tie strip holding the edges to the woodenarm at the end thereof to fall out, whereupon the tongues of the edgescome out from the grooves, causing separation of the edges from the arm.I have also found that the difiiculties observed in plastic edges ofT-squares made by molding or extruding proc esses but having their edgessmoothened by cutting are due to causes similar to those operating incases of plastic edges or arms made from the best quality cast sheets ofacrylic plastic material.

It is an object of the present invention to provide an improved methodof making a T-square, for use particularly but not exclusively with adrawing board, whereby the above difficulties and disadvantages areovercome and largely eliminated without introducing other problems orappreciably increasing the costs involved.

Another object of the invention is to provide an improved method ofmaking a T-square having transparent guiding edges made of acrylic resinmaterial and smoothened by removing some of the material, particularlywith the use of rotatable cutters or by sanding, means being provided toeliminate the inherent susceptibility of such edges to crumbling out ornicking as well as to chipping along their sharp line edges, even in thenormal use of the T-square, and breakage of their corners.

A further object of the present invention is to provide an improvedmethod of making a T-square having its arms made of cast sheet oftransparent acrylic resin having optically flat surfaces, providing anarm of undistortable transparency, and having guiding edges smoothenedby cutting or sanding, with such edges having greater resistance tocrumbling out and to nicking than conventional T-squares having theirentire arms, or only their 4 guiding edges, made of acrylic resin andsmoothened by cutting.

A still further object of the present invention is to provide animproved method of making a T-square having a head and an arm, said headand arm being secured together by a joint which eliminated anypossibility of loosening of the arm or the head, which looseningfrequently occurs in conventional T-square and creates a very annoyingcondition making the T-square virtually unuseable.

A still further object of the present invention is to provide animproved method :of making a T-squarehaving a head and an arm connectedtogether by a-joint which is strong and durable beyond the possibilityof failure under conditions of normal use, including conditions of usethat may be expected in schools.

A still further object of the present invention is to provide animproved method of making a T-square having at least its edges made ofacrylic resin forming corners at the free end of the arm, which corners,nevertheless, have high resistance to breaking off and to chipping off.

A still'further object of the present invention is to provide animproved method of making a T-square in which the use of wood iscompletely eliminated, thereby eliminating the disadvantages, such asthose due to absorption or drying out of moisture, differentcoefficients of expansion of wood and plastics, separation of plasticfrom wood at the cemented joints, wearing out of paint or varnishfinishes, and the like.

A still further object of the present invention is to provide a methodof making a T-square having a head of such configuration as to producemaximum practicable contact area between the contacting surfaces of thearm and of the head and thus to ensure the strongest possible jointproduced at said area by the method disclosed herein.

A still further object of the present invention is to provide animproved method of making a T-square made of acrylic plastic, whichT-square does not include any extruded or molded plastic partsdistorting the image underneath and causing eye strain.

A still further object of the present invention is to provide animproved method of making a T-square fabricated by cutting operationsfrom solid sheets or plates of acrylic resin, and having an improvedconstruction whereby the crumbling out of sharp edges is overcome andlargely eliminated in a simple and effective manner, thus producing astronger and more durable guiding edge.

A still further object of the present invention is to provide animproved method of making a T-square of the character specified in thepreceding paragraph, in which guiding edges including sharp line edgescausing undercutting and breakage of graphic pencil points areeliminated.

It is an added object of the present invention to provide an improvedmethod of making a T-square of the foregoing character which has onlytwo parts permanently secured together, and which thus is of thesimplest possible construction, which is pleasant to handle, convenientto use, and is relatively inexpensive to manufacture.

A still further object of the present invention is to provide animproved method of making a T-square of the foregoing nature in whichthe tendency of plastic T- squares to breakage of the blade adajcent theT-square head is virtually eliminated, thus producing in View of othernumerous advantages of my improved T-square an exceedingly durableinstrument capable of withstanding even the roughest usage to which theymay be normally subjected, such as that in schools.

A still further object of the present invention is to provide animproved method of making a T-square having a head and a solid plasticarm or blade secured thereto in any suitable manner, such as with theaid of screws or with the aid of an adhesive joint, in which T-squarethe transition from the rigid head to the flexible arm is made moregradual by stifiening and/or reinforcing the blade at the juncturethereof with the head and thus eliminating the weak spot possessed bysuch T-square at the place of juncture.

A still further object of the present invention is to provide animproved method of making a T-square having a head and a solid acrylicplastic arm secured thereto, in which T-square excessive flexibility orwhip in the arm is eliminated in the locality where such whip does notpresent any functional advantage but imposes stresses and strains on thearm which may prove to be beyond its strength.

A still further object of the present invention is to provide animproved method of making a T -square comprising a head and a solidacrylic plastic arm secured thereto, improved means being adapted toresist downward bending of the blade by being put in tension andcarrying a substantial portion of the load produced by the force causingsuch bending, as well as to resist upward bending of the arm by saidmeans operating to stiffen the blade immediately adjacent the T-squarehead.

A still further object of the present invention is to provide animproved method of making a T-square having a head and a solid acrylictransparent arm or blade, which T-square retains to a certain degree theaccustomed appearance of the T-square made of wood and having plasticlined guiding edges, and which, nevertheless, affords all of theadvantages of a fully transparent blade and does not have any of thedisadvantages of a wooden blade such as covering a material part of thedrawing and separating it into two parts, as is the case withconventional opaque Wooden arms.

A still further object of the present invention is to provide animproved method of making a T-square made of solid acrylic plastic andincluding a small piece of wood in the form of a veneer overlay whichpiece is so selected as to its size and is so disposed within thestructure of the T-square as to eliminate the disadvantages caused incon ventional T-squares by wood, such as stresses and strains imposed onplastic because of the difference between the coefficients of expansionof the respective materials, with the cumulative effect of suchdifference in theentire length of the arm causing cracking of theplastic.

A still further object of the invention is to provide an improved methodof making a T-square having a head and an arm overlying the head inwhich the disadvantages of conventional T-squares, such as annoying orpain-producing pressure on the users left hand is completely eliminatedby an expedient enhancing ornamental appearance of the T-square.

A still further object of the invention is to provide an improved methodof making a T -square including an arm, the free end of which hascorners made of plastic material, which corners are shaped to eliminatethe chipping and produceable in a simplified manner.

-A still further object of the present invention is to provide animproved method of making a T-square affording opportunities ofimparting to the T-square a striking ornamental appearance.

A still further object of the invention is to provide an improved methodof making a T-square having a head made of wood, plastic or any othersuitable material and an arm or blade made of solid acrylic or similarplastic material and secured to the head with the aid of screws orsimilar connectors, improved means being provided to eliminate thetendency of the arm of such a T-square to break at the head through theholes provided in the arm for the passage of such connectors.

Further objects and advantages of this invention will be apparent fromthe following description and appended claims, reference being had tothe accompanying drawings forming a part of this specification, whereinlike reference characters designate corresponding parts in the severalviews.

FIG. 1 is a top view showing an improved T-square embodying the presentinvention.

6 FIG. 2 is an elevational view of the T-square of FIG. 1. FIG. 3 is anenlargement of the portion of FIG. 2 encircled within the circle 3.

FIG. 3a is a fragmentary sectional view taken in the direction of thearrows on the section plane passing through the section line 3a3a ofFIG. 2.

FIG. 4 is a perspective view showing the free end of the arm of theT-square of FIGS. 1-3.

FIG. 5 is a perspective view illustrating the condition of the acrylicplastic material, such as used for the arm of my improved T-squares,after such material has been subjected to the action of a milling cutterfor smoothening of its edges.

FIG. 6 is a top view of the piece of material illustrated in FIG. 5.

FIG. 7 is a fragmentary end view taken in the direction of the arrow 7on the piece of the material illustrated in FIG. 5.

FIG. 8 is a view similar in part to FIG. 7 but showing an edge having achamfer of a modified construction.

FIG. 9 is an exploded view showing in section an edge of a still furthermodified construction.

FIG. 10 is a top view similar in part to FIG. 1 and showing a T-squareof a modified construction embodying the present invention.

FIG. 11 is a sectional view taken in the direction of the arrows on thesection plane passed through the section line 1111 of FIG. 10.

FIG. 12 is a fragmentary top view showing on an enlarged scale theportion of the fixed end of the T-square arm encircled in FIG. 10 with acircle identified by the numeral C FIG. 13 is a side view of the portionof the construction shown in FIG. 12.

FIG. 14 is a view showing on an enlarged scale in FIG. 10 by a circle Cthe free end of the arm of the T-square of FIG. 10.

FIG. 15 is a side view of the portion of the construction shown in FIG.14.

FIG. 16 is a fragmentary sectional view taken in the direction of thearrows on the section plane passed through the section line 16-16 ofFIG. 14.

FIG. 17 illustrates a modified form of overlay, namely, an elongatedstrip of fabric material adapted to be heat bonded to the plastic bladeof the T-square.

FIG. 18 is a view similar in part to FIG. 10 but showing a T-squareincluding a plastic arm secured to the head with the aid of screws andembodying the present invention.

FIG. 19 is a view similar in part to FIG. 18 but showing a still furthermodification thereof.

It is to be understood that the invention is not limited in itsapplication to the details of construction and arrangement of partsillustrated in the accompanying drawings, since the invention is capableof other embodiments and of being practiced or carried out in variousWays within the scope of the claims. Also, it is to be understood thatthe phraseology and terminology employed herein is for the purpose ofdescription and not of limitation.

While the present invention is illustrated and described with referenceto a T-square having an arm made of acrylic plastic, it will beunderstood that the invention is applicable to other plastic materialshaving similar physical properties, and particularly flexibility,hardness, and susceptibility to cracking as possessed by acrylicplastics. Accordingly, the term acrylic plastic as used in thespecification and claims is to be understood as referring to plasticmaterials of such nature.

Particularly, it shall be understood that in describing the Working orguiding edges of T-squares, which edges in my improved T-square includeboth the guiding surfaces of such edges as well as their surfacesproduced by chamfers, I refer to such edges as guiding edges. On theother hand, the sharp edges, which are produced by two surfaces meetingat an angle and producing such sharp edge, I term line edges. Thus, theterm line edges as used herein designates a geometric line at which suchsurfaces meet. Thus, an unchamfered guiding edge of a conventionalT-square has two line edges. On the other hand, the chamferedguiding'edge of my improved T- 'square has four or more'line edges.

In accordance with the present'invention I eliminate the use of wood asa material for the construction of T- squares. I provide an improvedT-square made of solid cast sheets or plates of acrylic resin plastic,with the head and the arm of the T-square being secured together by anon-loosening joint which is strong and durable beyond the possibilityof failure in normal use. The construction of the T-square, particularlyof the arm thereof, includes edges from which the material at the sharpor line edges thereof having the inevitable minute cracks caused by themilling cutter is removed, producing not only an edge free of suchminute cracks showing a tendency to grow but also an edge in which theincluded angle of its surface is much larger than 90 being,approximately 135. The free end of the arm is constructed in such amanner that breakage of its corners and chipping ofi? of the end edges,inevitable in conventional T-squares having acrylic plastic edges orentire arms, is reduced to a minimum, pro ducing a T-square uncamparablymore durable than conventional T-squares.

Referring specifically to the drawings, the T-square illustrated thereincomprises generally a head H and an arm A secured together as at toprovide a unitary construction. The head H is made by profilingfrom acast plate of acrylic plastic, preferably of solid black color. Theconfiguration of the head, while resembling that of a conventionalT-square head, is more rounded, particularly in places indicated by thenumeral 11. Therefore, the head H conforms much better to theconfiguration of the palm of the user and thus ensures a more convenienthold on the T-square in use. Furthermore, the configuration of thecurved edge of the head H is such as to increase the width of the headin the middle thereof, as compared with that of conventional T-squares,and thus to increase the area of the contacting surfaces of the head andof the arm for the purposes explained below. The ends of the head arerounded as indicated at 13, with the tangents to the end curves, attheir junctures with the straight guiding edge 14 of the T-square,forming angles a with said edge 14, with the vertices of said angles abeing directed toward the arm of the T-square. I prefer to make theseangles greater than 90 in order to protect the ends of the head frombreaking off or chipping. T-squares having said angles a equal toapproximately 110 give very satisfactory results and have a pleasantappearance.

A bevel, such as indicated by the numeral 12, made to a depth ofapproximately .060", eliminates the sharp corner along the curved edgeof the head H at the upper surface thereof, and also contributes to theadvantage of a more convenient hold on the head. A bevel similar to thebevel 12 may also be provided along the curved edge of the head H at thelower surface thereof, if desired. The bevel 15, which may be of thesame magnitude as the bevel 12, is provided also along the top of thestraight edge 14 for the purposes explained further below.

I prefer to make the head H somewhat thinner than wooden heads ofcorresponding size. For T-squares having arms less than 15" in length, Iprefer to use head 6" long and A to A" thick. T-squares having arms 15"to 28" give good results with heads 7 /2" long and A" thick. Heads 9"long and A" and even thick are sufliciently thick for T-squares havingarms between 28" and 30" in length. For T-squares with arms 30" and 36"long, heads 12" long and /8" thick give very good results.

The arm A is made of a cast sheet of acrylic resin. Such sheets are madeby casting them between two plates of optically fiat glass or othermaterial and, therefore, they have corresponding optically fiatsurfaces, producing 8 no distortion of the image underneath. On theother hand, molded and extruded plastics do not have 'such opticallyflat surfaces ensuring undistorted transparency, and their surfaces,being somewhat wavy and streaky, distort'the image underneath and causeeye strain in prolonged use. Arms of A" in thickness give excellentresults 'in lengths from 18" to 36" and 2" to 2%. in width. Arms of lessthan 18" may be made inwidth.

The guiding edges 20 and 21 of the arm A are smoothened with the aid ofrotatable cutters in suitable milling machines to produce smooth andstraight edges. The end edges 22 and 23 of the arm A are similarlysmoothened, and their corners are rounded on a suitable radius, such asas indicated at 2212 and 23a. v In accordance with the invention, inaddition to such rounding of the corners, the end 23 is also beveledpreferably' at a 45 angle, at both its top and bottom surfaces to thethickness of approximately .040" as is best shown in FIG. 4 at 23b. Itis of importance that the bevels 23b extend not only along the edge 23but also around the comers 23a, and extend, in effect, for a shortdistance into the longitudinal edges 20 and 21. I have found that merelyrounding the corners is wholly insufficient. Corners which are merelyrounded even on such a large radius as A" chip very badly, and after aperiod of use look about the same as the broken off square corners. Thisfact has been accepted in the art as'a necessary ev unavoidable in useof acrylic plastics. I have found that such chipping results from thefact that in a downward fall a T-square does not fall with its armstrictly perpendicular to the floor, and, therefore, the free end of thearm hits the floor with the arm being somewhat inclined. Therefore, thefree end of the arm almost always hits the floor with the sharp edge ofthe rounded corner. Chipping of the corners under such conditions takesout rather large pieces of plastic, and it is just as objectionable asbreaking off of square corners. The bevel 22b may be identical with thebevel 23b, but it is provided only at the upper surface of the T- squareand for different purposes. An important purpose served by said bevel22b is to eliminate shap edges at that end and to produce a morecomfortable hand hold on the T-squar'e head.

It is of great importance for the purposes of the present invention thatthe sharp line edges or corners of'the guiding edges 20 and 21 arechamfered for a depth of approximately .020" or more, both at the topand at the bottom of said guiding edges, thus producing a guiding edgeshown in FIG. 3a: wherein said chamfers are indicated by the numeral 25.Provision of such chamfers, in accordance with the invention, hasseveral exceedingly important advantages. In the first place the anglesb formed between the surfaces forming line edges 20a and 20b areincreased by virtue of the provision 'of such chamfers from to producinga guiding edge incomparably stronger and better resistant to nickingfrom mechanical causes, suchas hitting the edge in use on some object.Furthermore, each guiding edge in my improved T-square is, in effect,dulled. Therefore, it does not undercut the pencil point and does notcause its breakage in drawing long lines as is common with cut acrylicplastic edges of conventional T-squares and other drafting instruments,such as triangles. This particular advantage is disclosed in greaterdetail in my said copending applications. Chamfering the sharpsquareline edges of the arm also makes handling of theT-square much morepleasant. It is well known in the art that hand and finger cuts may becaused by sharp edges of cut acrylic plastic edges. I V

Of particular importance is the function of the provision of suchchamfer in removing the material along the line edges of the T-squarearm. For the reasons explained before I discarded the conventionalunderstanding that sharp line edges are not objectionable since,presumably,

or .10" thick and 1 /z'-"1%" they not only convey more clearly theimpression of precision in a new instrument, but lose their sharpnessafter several months of use. I have found that in cutting plastics ofincreased hardness and brittleness, such as acrylic plastics, anymilling cutter, no matter how sharp, produces separation of the materialby, in effect, tearing a portion of it off from the general mass. Suchtearing off of the material produces concentrated stresses particularlyat the sharp edges of the sheet and origination therein of minute crackswhich may extend only as much as .003 or .004" inwardly of the plasticsheet. However, such cracks have a tendency to grow and thus to extendfurther and further into the mass of the material. Should two suchcracks occur relatively close to each other and at a large angle, suchas 90, to each other, as indicated at 26 and 27 in FIGS. and 6, theysoon meet, whereupon the material between them crumbs out producing anick. Such nicks are often mistaken for those produced by hitting and,therefore, avoidable by careful use of the instrument. I have found thatorigination of such nicks is wholly independent of the conditions ofuse, and that such nicks may occur even in new instruments, particularlythose that have been stored for some time.

By providing the chamfer which may be approximately .020".030" in depth,I remove the material including the minute cracks, thus eliminating theabove described conditions. It should be appreciated at this point thatcutting the chamfer, such as 25, does not produce similar cracks alongthe lines 20a and 2012. First, the force exerted by the chamferingcutter on the plastic in the process of chamfering is extremely light ascompared to a force exerted by the cutter cutting through the entirethickness of the arm strip. Therefore, exertion of such force on theplastic mass does not produce distortion of said mass sufiicient torupture the material. Furthermore, the angles b being greatly increasedas compared to the angle 6 do not cause such high concentration ofstresses in the line edges, and whatever concentration may occur thereatis distributed between the two stronger edges and in the mass ofmaterial between them. It should be understood that bevels 22b and 23bhave a similar effect and advantages, in addition to those disclosedabove.

The chamfered edges may also be buffed, thereby producing a glossysurface at said chamfered edges and strengthening them by producing asurface possessing a property known as surface tension. Although all ofthe chamfers described above may be provided at 45 angles to the meetingsurfaces, it shall be understood that chamfers of other angles may alsobe used with success. Two chamfers producing a meeting line edge mayalso be utilized. For example, two chamfers as illustrated in FIG. 8 maybe provided at 30 angles 2 to the surfaces meeting along the line edge29, whereby all three internal angles 1 are increased to 150, producinga still stronger construction. Furthermore, a chamfering cutterproducing rounding of the corners rather than providing a straightchamfer may also be used with success. It shall be also understood thatif provision of the chamfer is desired both at the top and the bottomsurface of the head or the arm, particularly at the ends of the arm andalong the curved edge of the head, the same may be produced with the aidof a special cutter illustrated in FIG. 9 and designated therein by thenumeral 35, and producing an edge having two rounded corners. Such anedge is illustrated in FIG. 9 and is designated therein by the numeral36. Similar guiding edges may also be provided on the arm.

In accordance with the invention as illustrated in FIGS. 1-9, for thepurposes of securing the arm to the head I discard the use of separateconnectors, such as screws, bolts, and the like, although suchconnectors have been used not only for wooden heads and arms but alsofor connecting plastic arms to wooden or plastic heads. Use

of such connectors, unless combined with the features of constructionhereinafter disclosed, has a number of serious disadvantages in all ofsuch applications. I have also found that the place of juncture orcontact area between the head and the arm of conventional T-squares maynot provide sufficient area to produce a fully reliable joint by othermethods. Because of the increased width of my improved head at themiddle thereof, such area is suificiently increased and, therefore, acemented joint provided therein is not only much stronger than thatproduced with the use of mechanical connectors, but is also muchstronger than it could be otherwise, irrespective of the method.

My improved joint is fully non-loosening. This is a very importantadvantage, since loosening of the joint between the head and the arm maycause the user to operate for considerable length of time, prior to theT-square becoming unuseable, with a T-square in which the arm may moveslightly and unnoticeably with respect to the head and produce a mistakein a drawing which may be rather serious, but which, for a considerablelength of time, could be attributed to some other cause.

I have also found that the opinion of those skilled in the art to theeffect that such a joint is not practical in a T-square and should notbe attempted is based on a misconception. Such a misconception may bedue to the fact that in cases of molded parts the slight waviness oftheir mating surfaces causes such a large number of places in the jointwhere air is entrapped or where no contact occurs in cementing as toproduce an unreliable joint. I have also found that this clifficultyincreases with increasing the area of the joint and may make such ajoint prohibitive even in a relatively small area such as presented in aconventional T-square, making further increasing such areaobjectionable. However, by using an increased area produced by thecontact of two optically flat surfaces I produce a superior andunexpectedly strong joint.

I prefer to produce such a joint by using a solvent effective todissolve and soften the surface of acrylic resin plastic and to produce,in effect, intermingling of the molecules of the two parts. Such anexpedient produces a joint approaching in strength that of the virginmass of the material. Actual experiments with my improved T- square haveshown that loosening of such a joint is an impossibility, and that inattempts to separate the parts, either the head or the arm breaks first.

The bevel 15 provided along the straight guiding edge of the head H, andextending in the assembled condition of the T-square both under the armA and on both sides thereof, performs in my improved T-square threeimportant functions, as described below.

First, it clears any nicks that may be found along the line edge of theguiding edge of a drawing board, as mentioned. Such nicks or raisedspots may be produced by separation of splinters of wood at said edge inwooden drawing boards, and by protrusion of the metal at the nicks ofmetal guiding edges provided in some T-squares.

The second important function of this chamfer is to eliminate theobjectionable effect of the small bubbles or tears of plastic producedby the solvent squeezed out from between the contacting plastic surfacesin producing the joint. In some cases the solvent, which has alreadydissolved some of the plastic material, comes out at the guiding edge ofthe head under the arm in the form of a rounded tear drop. Such a ballor drop may retain some of its shape and upon solidification of thejoint remain as a raised spot under the arm, protruding beyond theguiding surface of the guiding edge. In addition, such a tear drop,which upon dissolving some of the plastic material acquires theproperties of a cement, retains in itself a piece of foreign matterwhich freezes in place. Such a piece or crumb of foreign material formsa protrusion causing rocking of the head on the edge of the drawingboard. While such rocking may be hardly noticeable in the head, it ismultiplied manifold in the arm, causing rocking of its free end for aconsiderable distance and thus producing an unreliable T-square. Byvirtue of the provision of bevel 15, such protrusions are housed withinthe space of the recess and do not adversely affect precision of theT-square. For this particular purpose of the invention, the bevel 15should extend at least under the arm and for only a short distance onboth sides thereof. It may also be in the form of a square or cornerrecess. However, I prefer to provide such bevel at a 45 angle and alongthe entire length of the guiding edge of the head H in order to increasethe included angles of its two line edges to 135 and thus greatlyimprove the strength of the guiding edge. This is the third advantage,and it is of importance.

T-squares of solid acrylic plastic constructed as disclosed above inspite of their much lighter construction proved to be greatly superiorto wooden T-squares with acrylic plastic lined edges, both with respectto durability and their convenience in use. Contrary to expectations ofthose skilled in the art, they withstand rough school use withnegligible breakage. By tracing virtually each and every actual case ofbreakage within the region of predominant use of applicants equipment,applicant has found that wherever the breakage of his T-square occurred,it was generally due to some extraneous circumstances such asintentional abuse, repeated dropping on cement floors, particularlyearly in the morning after the T-square had become particularly cold inan unheated room during a cold winter night. By carefully tabulating thecases of such breakage, applicant found that it occurred most frequentlyin the arm immediately adjacent the head.

The improvements disclosed below are intended to reduce to a minimum thepossibility of such breakage first by decreasing the tendency of the armto bend and thus cause high localized stresses and strains in its outerfibers and particularly in the sharp corners thereof. My co-pendingapplication, Ser. No. 'l78,868, disclosed an improvement decreasingoccurrence of localized stresses when such bending occurs due to variouscauses, and has proved to be a very valuable expedient. The presentapplication discloses a further improvement whereby the tendency of thearm to bend or occurrence of loads resulting in localization of stressesis reduced, thus reducing the possibility of occurrence of suchlocalized stresses. Accordingly, the improvement disclosed below can beused in combination with the expedients disclosed in my said copendingapplication or independently thereof.

FIG. illustrates a T-square constructed in accordance with theinvention. The T-square illustrated therein comprises a head H and anarm or blade A which, with the exception of additional featuresdescribed below in detail, may be the same as those in the T-squareillustrated in FIGS. 1-9. The joint between the head H and the arm A ofthe T-square may also be of the same nature to provide a bond throughthe entire area of contact between the head and the blade, with orwithout rounding the corners of such bonded area as disclosed in my saidco-pending application, Ser. No. 178,868.

In accordance with the invention, at the top surface of the arm A at thefixed end thereof there is provided an overlay generally indicated bythe numeral 40 and extending longitudinally of the arm from its bevelededge 41 over the guiding edge 42 of the head and for a substantialdistance toward the free end of the arm. The exact length of the overlay40 depends on the size of the T- square. Generally, it is desirable toprovide an overlay extending beyond the guiding edge 42 forapproximately the width of the head or somewhat further in order toprovide a substantial bonded area between the piece 40 and the arm A.The exact dimensions may be modified for the purpose of giving theoverlay piece 40 and, therefore, the entire T-square a pleasingappearance. The width of the piece 40 is smaller than the width of thearm A in order to leave transparent edge portions 43 similar inappearance in that portion of the arm to those of the wooden arms withplastic lined edges. The right hand end of the T-square may be madetapering as indicated at 44. With a T-square having an arm 24" long and2% wide, an overlay piece 1%" by 5 /2" proved to be very satisfactory.

It is of importance that the overlay piece 40 can resist a substantialload in tension. It is desirable, therefore, that the material of theoverlay piece have greater resist-ance in tension than the plasticmaterial of the arm, i.e. has a greater coefficient of elasticity thanthe material of the arm in order for the piece to assume a substantialportion of the load imposed on the arm by a force tending to bend thearm downwardly. Such overlay may be a piece of strong fabric, a piece ofwood veneer, or of another plastic. If wood veneer is used, it isimportant that the wood grain of the veneer runs longitudinally of thearm to add a further advantage of being resistant to bending of the armupwardly. The thickness of the wood veneer should be selected in view ofits resistance to tension along the grain. I found that wood veneerapproximately .040" thick is very satisfactory.

It is also of importance that connection of the overlay piece 40 withthe arm A be substantially throughout the entire area of the contact andbe as uniform throughout such area as practicable. Various glues orcements may be used for such purpose. A solvent suitable for the plasticmaterial of the arm may also be used, providing that it does not causecrazing of the plastic. I found that various rubber base cementscommonly known as contact glues, give good results. Where a fabric piecesuch as the one designated by the numeral 45 in FIG. 17 is used, thesame may be treated on its underside with a suitable glue and heatbonded to the blade. The use of ready made strips of such natureintended for repairing clothing and commonly known as iron-on patchesmay also be used.

I prefer the use of wood veneer with the grain running longitudinally ofthe arm since such veneer atfords strong resistance in tension withlittle stretching and is also sufficiently rigid in the thicknessmentioned above to resist upward bending of the arm. It should beappreciated that downward bending of the arm is resisted by putting intension the overlay piece, be it of wood or fabric, and by putting thebonded area thereof in shear, thus disturbing the tension on the overlaypiece throughout the entire area thereof. On the other hand, the upwardbending of the arm would tend to put the overlay piece in bending andcompression. It is obvious that a fabric piece cannot resist bendingand, therefore, such pieces are less advantageous in this particularrespect.

Furthermore, wood veneer can be finished in an attractive manner to showwood grain and imparts to the T-square, at least in the vicinity of itshead, the accustomed appearance of a wooden T-square having a plastichead and transparent plastic edges but leaving the working portion ofthe blade fully transparent, thus eliminating the disadvantage ofopaqueness of wooden T-squares. Walnut, mahogany, birch, and many othertypes of veneer are now staple items in the trade and are relativelyinexpensive. An overlay of wood veneer stiffens the arm where itsbending tends to be the greatest and resists bending of the arm both inthe upward direction as well as in the downward direction. It also makestransition from the rigid part of the T-square to the flexible partthereof more gradual. Tapering of the overlay piece at its right handend makes such transition in the arm itself also more gradual and,therefore, is advantageous from the standpoint of strength.

Considerable problems have been experienced in the past because of theditficulty of holding conventional T-squares at their heads. The comersof the fixed end of the T-square arm pressing on the hand of the userduring prolonged use of the T-square cause painful spots requiringholding the T-square head only with the fingers rather than with theentire hand, all in the belief that such an unnatural and inconvenienthold is the solution for such problem. Such belief was caused by thefact that attempts to bevel the end edge of the arm, no matter howheavy, or rounding the corners thereof not only did not prove effectivebut aggravated the problem. FIGS. 12 and 13 illustrate an improvedfeature of construction completely eliminating such a difiiculty in anexceedingly simple and effective manner. In accordance with theinvention the end edge of the arm is first beveled, as indicated at 41in FIG. 10, whereupon the longitudinal sides of the arm at such cornersare beveled with a 30 beveling cutter at an angle between 20 and 30 tothe sides of the arm to produce triangular areas 47, as indicated inFIGS. 10, 12 and 13. With such a construction the user can handle theT-square for long periods of time without noticing any inconveniencewhatsoever.

FIGS. and 14-16 illustrate a modified construction of the free end ofthe arm intended to attain the advantages of the construction of FIG. 4.While the construction of FIG. 4 proved exceedingly effective, itsproduction involves a number of complications and may require severaloperations. The construction shown in FIGS. 10, and 1416 produces a freeend of the arm of three-dimensional curvature to attain the advantagesof the construction of FIG. 4, with such configuration susceptible ofbeing produced in a single operation.

As can be seen in FIG. 14, the free end of the arm A has configurationincluding a substantially straight portion 50 and angular portions 51merging with said edge 50 at well rounded portions 52. As can be seen inFIGS. 15 and 16, such curvilinear configuration is of a rounded crosssection throughout its entire length, which may be attained with the useof a cutter such as shown in FIG. 9. A particular advantage of thisconstruction is that it can be produced in one operation, with the armbeing guided by a template, and produce proper blending in spite of thefact that due to manufacturing variations arms may vary as much as andeven more in their width. With the use of configurations includingblending-in arcs, the difference in width of the arms due tomanufacturing variations makes proper blending of the corners with thestraight portions of the arm very difficult and produces defects such asdips, and the like. The construction of FIG. 15 eliminated such adisadvantage.

It should be appreciated that the fixed end of the arm may also be givena similar configuration. However, in stead of a rounded cross section,its cross section should be substantially square at the bottom Withheavy bevel at the top.

FIG. 17 illutrates an overlay piece made of substantially unstretchablefabric and having its underside treated with a suitable glue for hotbonding the piece to the T-square to be disposed thereon in a mannersimilar to the piece 40 of FIG. 10. Such fabric piece can be cut offfrom rolls of fabric tape commonly sold for the purpose of clothesrepairing. Since such fabric pieces do not impart stiffness to the arm,tapering the end thereof as shown at 44 in FIG. 10 is less important,and merely cutting off the sharp corners thereof as illustrated at 48may be sufficient.

FIGS. 18 and 19 illustrate an improved T-square in which a solid orone-piece acrylic plastic arm 55 is connected with the aid of threadedconnections, such as screws 57, to the head 56, which head may be ofplastic, wood, or other suitable material. As mentioned above, withconventional constructions breakage of such arms through the screw holesmade such constructions virtually impractical. In accordance with theinvention this difficulty is eliminated by a piece of pliable materialof high tensile strength, such as fabric, wood veneer, and the likebonded through its entire area to the head. In FIGS. 18 and 19 suchoverlay pieces are designated by the numerals 58 and 59, respectively.The piece 58 extends over the joint between the arm 55 and the head 56through a distance approximately equal to the width of the head. Inother words, the overlay piece 58 extends over the fixed end of the arm55 in its portion overlying the head 56. The width of the piece 58 maybe substantially the same as that of the arm 55. It will now be seen inview of the foregoing that tightening of the screws 57 would cause thescrew heads to bear not against the surface of the plastic directlyproducing the danger of cracking the plastic, but over the surface ofthe overlay piece cushioning and distributing the pressure moreuniformly. The fixed end of the arm 55 is also greatly reinforced by theprovision of the piece 58 bonded thereto. The piece 59 is similar in itsshape to piece 40 of the construction of FIG. 10 and is also bonded tothe surface of the arm A throughout substantially the entire area ofsuch piece, with the wood grain running longitudinally of the arm.

It will be understood that with some cements bonding of the overlaypiece to the plastic may not produce a continuous or unbroken bond areabut a multiplicity of small bonding spots distributed through the areaof contact. While the occurrence of dry spots between such bonded spotsdoes not decrease the strength of the joint, it, nevertheless, affordsattainment in a substantial degree of the advantages disclosed above.However, full advantage of such expedient may be better realized byimproving the bond between such surfaces to produce a substantiallyunbroken bonded area without interruption or dry spots.

It will also be understood that the constructions disclosed herein areexceedingly advantageous for repairing solid acrylic T-squares andparticularly T-squares such as illustrated in FIGS. 1-9 should the samebreak adjacent their heads. To repair such a T-square, the surfaces ofthe arm at the break are first covered with a suitable cement, glue orsolvent and are brought together, with the guiding edges as well as thetop and bottom surfaces of the arm being properly aligned. After thejoint is dry, such repair may be sufficient by itself. Some brokenT-squares have been so repaired and have been used with success for along time. However, bonding an overlay piece such as 40 or 45 over thebreak in the manner explained above will greatly reinforce the T-squareat the break joint and will bring the strength of the repaired T-squarevirtually to that of a new T-square. T-squares having one-piece acrylicplastic arms secured to a head with the aid of screws should they breakat the arm through the screws holes may be similarly repaired to producethe constructions shown in FIGS. 18 and 19.

In view of the above disclosed construction, the objects of the presentinvention listed above and numerous additional advantages are attained.

I claim:

1. In a method of making a T-square to attain full transparency of itsarm, functional effectiveness and to I prevent rapid breakage thereof inuse, the steps of providing a cast and annealed sheet of transparentacrylic plastic material, with its surfaces having been cast againstpolished glass plates to attain optical flatness of said surfaces,sawing an elongated strip of said material, milling the elongated sidesand at least one end of said strip to square and smoothen the sides ofsaid strip and thus to provide smooth guiding surfaces, chamfe-ring allsharp line edges formed on said strip by milling for approximately .020"to remove the material thereof subject to initiations of minutes crackstherein and growing of said cracks inwardly of the arm therefrom, andpolishing the chamfered edges to restore the luster and the surfacetension therein to prevent origination and growing of minute cracks fromthe narrow surfaces and from the double edges produced by suchchamfering.

2. The method defined in claim 1 and including further steps ofconnecting the arm so produced to a T-square head, trimming the secondend of said strip to bring the same to final length size, eliminatingthe sharp corners at said end with use of cutter means producing convexend cross section and thus off-setting the top line edges at the end ofthe arm inwardly of the arm, with such cross section turning toward thehead of the T-square at both corners of said end, and polishing said endto restore the luster and the surface tension in the surfaces of saidend and thus to prevent origination of minute cracks at the end surfacesand their extension inwardly of the arm on impact.

3. The method defined in claim 2, and including the step of machiningthe other end of the arm to provide a bevel on the top surface thereofonly to the extent of at least half of the thickness of the arm at theend and around the two corners thereof.

4. The method defined in claim 1 and including the further steps ofproviding a cast and annealed sheet of acrylic plastic materialapproximately twice as thick as the sheet for making the arm, sawing apiece of said material in the form of a T-square head with machiningallowance, profile milling all edges of said head piece to provide astraight guiding surface thereon and to impart to said piece the desiredform, relieving all sharp line edges on both sides of said piece toeliminate said edges and to set back the top and bottom line edges ofsaid piece inwardly thereof, polishing all milled and beveled edges torestore the luster and the surface tension thereof, and laminating thebeveled edge of the arm and the head piece so produced by solvent jointto produce a unitary structure, with the trimming of free end of the armbeing done after completion of the laminating process.

5. In a method of making a T-square to attain full transparency of itsarm, functional effectivenessand to prevent rapid breakage thereof inuse the steps of providing a cast and annealed sheet of transparentacrylic plastic material, with the surfaces of said sheet having beencast against polished glass plates to attain optical flatness of saidsurfaces, sawing an elongated strip of said material, milling theelongated sides and at least one end of said strip to square andsmoothen the sides of said strip and thus to provide smooth guiding sidesurfaces, chamfering sharp line edges formed on said strip because ofmilling for approximately .020" in order to remove the material thereofsubject to initiations of minute cracks therein and growing of saidcracks inwardly of the arm therefrom, providing a cast and annealedsheet of acrylic plastic material at least twice as thick as the sheetfor making the arm, sawing a piece of said material in the form of aT-squre head with machining allowance, profile milling all edges of saidheadpiece to impart to said piece the desired form and to provide astraight guiding surface thereon, relieving all sharp line edges on bothsides of said piece to eliminate said edges, laminating the finished endof the arm and the head by solvent joint to produce a unitary structure,trimming the free end of the a-rmto bring the length of the arm to itsfinal dimension, with said trimming being done after the step oflamination and with the use of cutter means producing convex end crosssection, and cutting off the corners of the free end of th arm with theuse of the same cutter means.

6. The method defined in claim 5 and including th additional steps ofproviding a piece having at least one layer of wood veneer, said piecehaving thickness smaller than that of the arm, and with its width beingnarrower than that of the arm, and length equal to approximately two andone-half widths of the head, laminating said piece to the upper surfaceof the arm to leave the side edges of the arm transparent and with saidpiece to extend substantially from the laminated end of the arm over thejoint of the head with the arm, the guiding surface of the head and theportion of the arm adjacent to said guiding edge.

7. The method defined in claim 6, with the layer of wood veneer of saidpiece being the upper layer thereof and having a finish bringing out thegrain thereof, said grain to run longitudinally of the arm.

References Cited UNITED STATES PATENTS 583,058 5/1897 King 33-1122,315,004 3/1943 Painter 331 12 2,562,348 7/1951 Bowser, 33-1072,958,939 11/ 1960 Turner 29407 3,103,071 9/1963 Dolgorukov 33112 THOMASH. EAGER, Primary Examiner.

1. IN A METHOD OF MAKING A T-SQUARE TO ATTAIN FULL TRANSPARENCY OF ITSARM, FUNCTIONAL EFFECTIVENESS AND TO PREVENT RAPID BREAKAGE THEREOF INUSE, THE STEPS OF PROVIDING A CAST AND ANNEALED SHEET OF TRANSPARENTACRYLIC PLASTIC MATERIAL, WITH ITS SURFACES HAVING BEEN CAST AGAINSTPOLISHED GLASS PLATES TO ATTAIN OPTICAL FLATNESS OF SAID SURFACES,SAWING AN ELONGATED STRIP OF SAID MATERIAL, MILLING THE ELONGATED SIDESAND AT LEAST ONE END OF SAID STRIP TO SQUARE AND SMOOTHEN THE SIDES OFSAID STRIP AND THUS TO PROVIDE SMOOTH GUIDING SURFACES, CHAMFERING ALLSHARP LINE EDGES FORMED ON SAID STRIP BY MILLING FOR APPROXIMATELY .020"TO REMOVE THE MATERIAL THEREOF SUBJECT TO INITIATIONS OF MINUTES CRACKSTHEREIN AND GROWING OF SAID CRACKS INWARDLY OF THE ARM THEREFROM, ANDPOLISHING THE CHAMFERED EDGES TO RESTORE THE LUSTER AND THE SURFACETENSION THEREIN TO PREVENT ORIGINATION AND GROWING OF MINUTE CRACKS FROMTHE NARROW SURFACES AND FROM THE DOUBLE EDGES PRODUCED BY SUCHCHAMFERING.